1. Field of the Invention
The present invention relates to a rear projection screen comprising a Fresnel lens and a lenticular lens.
2. Related Art
Rear projection screens that comprise Fresnel lenses for refracting rays projected from an imaging source to make them almost parallel with each other, and lenticular lenses for scattering projected rays to form an image have conventionally been known as screens useful for rear projection type TVs and the like.
Further, among the conventional rear projection screens, there has been known a screen in which projected rays are diffused in the horizontal direction by a lenticular lens sheet for horizontal diffusion, and in the vertical direction by both a diffuser and a lenticular lens sheet for vertical diffusion. It is noted that the diffusion angles of these lenticular lens sheets are generally uniform over the entire screen surface.
In the conventional rear projection screens, the diffusion angle is uniform over the entire screen surface, so that there is such a problem that the phenomenon of shading, which will be described hereinafter, tends to occur.
FIGS. 12A, 12B and 12C are views for illustrating the phenomenon of shading that occurs on a rear projection screen. In these figures, FIG. 12A is a view showing how those rays projected from an imaging source 20 are observed on a rear projection screen 10. In this figure, the intensity and direction of diffusion at three points a, b and c on the rear projection screen 10 are indicated by the length and direction of an arrow, respectively. Further, FIGS. 12B and 12C respectively show the luminance distributions on the rear projection screen 10, obtainable by observing the screen from observation points p1 and p2.
As can be understood from FIGS. 12A and 12B, when the rear projection screen 10 is observed from the front (observation point p1), the luminance at the marginal part (edges a and c) of the rear projection screen 10 is lower than that at the central part b of the rear projection screen 10, so that the marginal part of the rear projection screen 10 is perceived as dark. Further, as can be known from FIGS. 12A and 12C, when the rear projection screen 10 is observed from a position (observation point p2) deviated from the position in front of the screen, the luminance at the edge a which is near the observation point p2 is highest, and this luminance gradually decreases as the distance from the edge a increases toward the central part b and then toward the edge which is at the opposite of the edge a. Therefore, those parts on the rear projection screen 10 which are apart from the observation point p2 are perceived as dark.
In such a rear projection screen 10, the diffusion angle in the vertical direction is generally set to be narrower than that in the horizontal direction. Therefore, the phenomenon of shading tends to occur in the vertical direction on the rear projection screen 10, and the brightness on the rear projection screen 10 has thus lacked uniformity.
A method in which the focal length of the Fresnel lens constituting the rear projection screen 10 is decreased has been known as a method for reducing the occurrence of such a shading phenomenon. By this method, the point of focus on the observation side of the Fresnel lens (i.e., the observation side of the rear projection screen 10) is made closer to the lens, so that, in FIG. 12A, a ray of light projected on the marginal part (edges a and c) of the rear projection screen 10 emerges by being inclined to the inside. For this reason, in this rear projection screen 10, increased luminance is observed on the marginal part (edges a and c) of the rear projection screen 10 when the screen is observed from the front (observation point p1). Moreover, even when the screen is observed from a position (observation point p2) deviated from the position in front of the screen, improved uniformity of brightness is attained on the rear projection screen 10 because decreased luminance is observed at the edge a which is near the observation point p2, while increased luminance is observed at the edge c which is apart from the observation point p2.
However, in such a rear projection screen 10, when the focal length of the Fresnel lens constituting the rear projection screen 10 is decreased, the difference among the exit angles of imaging rays of three colors projected from different positions on the imaging source 20 becomes large. When the difference in exit angle in terms of vertical direction against which the diffusion angle of the rear projection screen 10 has been set narrow becomes large, color shading tends to occur on the rear projection screen 10. When this color shading is taken into consideration, it is difficult to make the focal length of the rear projection screen 10 on the observation side shorter than approximately 10 m. Therefore, it has been impossible to reduce the occurrence of the shading phenomenon without bringing about color shading.
The present invention was accomplished in the light of the foregoing. An object of the present invention is therefore to provide a rear projection screen capable of attaining the uniformity of brightness over the entire screen surface without bringing about the the lowering of luminance on the marginal part of the screen surface, and the nonuniformity of luminance on the screen surface even when the screen is observed from a position deviated from the front.
The present invention is a rear projection screen comprising a Fresnel lens, and a lenticular lens, having diffusion properties that vary continuously between a central part and an edge of the screen surface.
In the present invention, the above-described lenticular lens is preferably such that its diffusion angle continuously increases as the distance from the central part toward the edge side on the screen surface increases and its direction of diffusion is gradually inclined to the central part side or to the edge side as the distance from the central part toward the edge side on the screen surface increases.
Further, it is preferred in the present invention that the above-described lenticular lens comprises a plurality of convex lenses and that the shapes of these plural convex lenses continuously vary as the position of the convex lens gets apart from the central part toward the edge on the screen surface. Specifically, it is preferable to make the heights of the above-described plural convex lenses continuously increase as the position of the convex lens gets apart from the central part toward the edges on the screen surface. Further, among the above described plural convex lenses, a convex lens arranged at the central part of the screen surface is preferably in such a shape that the deviation of the top of the convex lens from the center of the same is almost zero; and it is preferable that this deviation of the top of the convex lens from the center of the same continuously increases as the position of the convex lens moves from the central part toward the edges on the screen surface. Furthermore, each convex lens described above has a cross section preferably in such a shape that two circular arcs with different radii are connected. Among the above-described plural convex lenses, a convex lens arranged at the central part of the screen surface is preferably in such a shape that the proportion of the circular arc with a larger radius is higher, and it is preferable that the proportion of the circular arc with a smaller radius increases continuously as the position of the convex lens moves from the central part side toward the edge side on the screen surface. Moreover, it is preferable that the above-described plural convex lenses be arranged with a constant pitch.
In the present invention, it is preferable that the above-described lenticular lens be for vertical diffusion.
According to the present invention, since the diffusion properties of the lenticular lens continuously vary with the distance from the central part toward the edges on the screen surface, it is possible to attain the uniformity of brightness over the entire screen surface without bringing about the lowering of luminance on the marginal part of the screen surface, and the nonuniformity of luminance on the screen surface even when the screen is observed from a position deviated from the front.